(1) Field of the Invention
The present invention generally relates to evaporative fuel control apparatus, and more particularly to an evaporative fuel control apparatus of an internal combustion engine for feeding fuel vapor from a fuel tank to an intake system through a purge passage in which a purge control valve is provided.
(2) Description of the Related Art
Conventionally, in an internal combustion engine, an evaporative fuel control apparatus is known, which stores fuel vapor from a fuel tank in activated carbon in a canister and feeds the stored fuel vapor from the canister into an intake system of the internal combustion engine. The feeding of fuel vapor into the intake system is called hereinafter the purging of fuel vapor. Also, there is a known internal combustion engine which has a fuel injection control part performing a feedback control to control the air-fuel ratio of the air fuel mixture fed back from the internal combustion engine to converge toward the stoichiometric air-fuel ratio. For example, Japanese Laid-Open Patent Application No. 63-289243 discloses a prior fuel injection control apparatus which corrects, during the purging of fuel vapor, the amount of fuel being injected to a combustion chamber, in addition to performing the feedback control of the air-fuel ratio described above. The amount of the correction to correct the amount of fuel injected is determined in response to the concentration of fuel vapor in the intake mixture. The concentration of fuel vapor is calculated from the average of a feedback correction factor FAF with respect to the air-fuel ratio. The correction of the amount of fuel injected which is made by the fuel injection control apparatus owing to the amount of fuel vapor purged allows accurate follow-up control of the air-fuel ratio. A description of the feedback correction factor FAF is disclosed, for example, in the U.S. Pat. No. 4,841,940 assigned to the same assignee as the present invention, and the disclosure of this patent regarding the term "feedback correction factor" is hereby incorporated in this specification for clarity.
In the case of the prior fuel injection control apparatus described above, when the concentration of fuel vapor in the intake mixture is high, the amount of the correction due to the fuel vapor purging relative to the amount of fuel injected becomes too great, and accordingly the amount of fuel injected after the fuel vapor purging becomes too small. However, the minimum level of the amount of fuel injected into a combustion chamber of the internal combustion engine is predetermined, that is, the amount of fuel injected has to be invariably higher than such a lower limit of the fuel injection amount. The fuel injection amount hereinafter means the amount of fuel which is fed by a fuel injector into a combustion chamber of the internal combustion engine. Thus, the prior fuel control apparatus has a problem in that when the fuel vapor concentration is extremely high, the amount of the correction to correct the fuel injection amount due to the fuel vapor purging is not accurately controlled, thereby the fuel injection amount is sometimes lower than the predetermined minimum level and the air-fuel ratio is not appropriate for the internal combustion engine.
One solution to the above mentioned problem is an evaporative fuel control apparatus of an internal combustion engine in which both the amount of fuel injected and the amount of fuel vapor purged are suitably corrected in response to the concentration of fuel vapor, so that the air-fuel ratio is maintained at the stoichiometric air-fuel ratio invariably, even when the concentration of fuel vapor is very high. Therefore, one aspect of the present invention is directed to an evaporative fuel control apparatus of an internal combustion engine having such a correction capability.
In the case of the evaporative fuel control apparatus having the above mentioned correction capability, an intake air pressure is indicated by an output signal of a sensor provided downstream of a purge port in the intake passage, and a canister in a fuel supply system communicates with the purge port via a purge control valve or vacuum switching valve at an intermediate portion of a fuel vapor supply conduit. In the above mentioned fuel control apparatus, when the air fuel mixture is tending to be too lean, the amount of fuel vapor purged through the intake passage to correct the amount of fuel injected is controlled to increase.
However, in a case of an internal combustion engine in which the flow rate of intake air is measured by a signal from an air flow meter provided upstream of the purge port in the intake passage, it is difficult to perform accurate control of the air-fuel ratio. That is, if the amount of fuel vapor purged into the intake passage is controlled to increase when the air-fuel ratio of the mixture is detected to be too small, the air fuel mixture becomes excessively lean when the concentration of fuel vapor in the intake mixture is low. Therefore, there is a problem in that the fuel control apparatus of the internal combustion engine of the type described above cannot always achieve accurate control of the air-fuel ratio when the air fuel mixture is particularly lean.